G4 (Severe) Geomagnetic Storm of September 24, 1998
A G4 (severe) geomagnetic storm began September 24, 1998, reached Kp 8+ and lasted 21 hours, with a daily Ap as high as 117; at that Kp the aurora may be seen from about 43 degrees geomagnetic latitude.
The Kp through the storm
Began Sep 24, 21:00 UTC, peaked in the block from Sep 25, 06:00 UTC, ended Sep 25, 18:00 UTC.
The storm
- Peak Kp
- 8+ (8.33)
- NOAA level
- G4 Severe
- Duration
- 21 hours
- Highest daily Ap
- 117
- Highest 3-hour ap
- 236
- Rank since 1932
- 106 of 4,139
How far south the aurora could have been seen
At Kp 8+ NOAA’s scales put the aurora’s reach at about 43 degrees geomagnetic latitude: 35°N in the central US, 34°N in the eastern US, 37°N in the western US and 44°N in northern Europe.
NOAA says a G4 storm’s aurora has been seen as low as Alabama and northern California.
US cities within reach, the southernmost first
- San Francisco, CA 43.3° geomag.
- Little Rock, AR 43.3° geomag.
- Oakland, CA 43.4° geomag.
- Modesto, CA 43.4° geomag.
- Huntsville, AL 43.6° geomag.
- Stockton, CA 43.7° geomag.
- Oklahoma City, OK 43.7° geomag.
- New South Memphis, TN 43.8° geomag.
- Memphis, TN 43.9° geomag.
- Santa Rosa, CA 44.0° geomag.
106 US cities of 150,000 people or more sit at or poleward of that line. Low on the northern horizon under a dark, clear sky; not a record of sightings.
Day by day
The daily Ap is the mean of the day’s eight three-hour ap values.
| UT day | Daily Ap | Highest Kp that day |
|---|---|---|
| September 24, 1998 | 28 | 6 |
| September 25, 1998 | 117 | 8+ |
Flares before it
| Peak (UTC) | Class | Region | Before the storm began |
|---|---|---|---|
| Sep 23, 07:13 UTC | M7.1 | NOAA 8340 | 38 hours |
The strongest M and X class flares GOES recorded from four days before the storm began to its end, the window in which a coronal mass ejection launched with a flare usually arrives. A flare in this list is context, not proof of the cause: many storms come from coronal holes, and many flares launch nothing toward Earth.
More
The storm before, Sep 18The storm after, Oct 1Every storm of 1998The strongest on recordThis day in weather historyTonight’s aurora forecast
Sources
Kp and ap from GFZ Helmholtz Centre for Geosciences, Geomagnetic Observatory Niemegk, licensed CC BY 4.0: Matzka, J., Stolle, C., Yamazaki, Y., Bronkalla, O. and Morschhauser, A., 2021, The geomagnetic Kp index and derived indices of geomagnetic activity, Space Weather, doi:10.1029/2020SW002641; data doi:10.5880/Kp.0001. Storms, levels and aurora reach worked out by WeatherOverTime from those values and NOAA’s space weather scales. Definitive through August 31, 2026; later values are preliminary. Rebuilt 2026-09-24. Flares from NOAA NCEI’s GOES X-ray flare reports (1975 to 2017) and SWPC’s edited solar event lists, public domain; classes as NOAA recorded them at the time.